This invention relates the art of electrical measurements, and more particularly to an electrical measuring method and apparatus which provides for the voltage measurement and pre-charging of an isolated and/or remote surface or structure such as the ion collecting plate element of a floating plate ion monitoring system.
Floating plate ion monitoring systems are typically used to measure the effectiveness and efficiency of room air ionization systems employed in the semiconductor manufacturing process industry to reduce or eliminate charge accumulation on charge sensitive semiconductor or LCD (liquid crystal devices) during the manufacturing process.
As conventionally known, charge accumulation on charge sensitive semiconductor elements such as mosfet gates arrays, digital memory or logic elements, or LCD devices using TFT device (thin film transistors), if not controlled or eliminated will cause immediate destruction or early life field failure of the semiconductor junctions of these devices.
Air ions, both positive and negative, are typically used to flood areas where semiconductor devices are being processed to provide a pool of mobile air ion charges which can be attracted by the undesirable charges associated with the semiconductor devices under process to effectively cancel them to zero net charge.
The typical floating plate ion monitoring system plays an important role in the air ion control system by providing a means of measuring the quantity of mobile air ions of each polarity being produced by an air ionization system, as well as providing a measure of the ability of the produced air ion field to hold various spatial areas associated with the semiconductor process line at a zero or near zero voltage level.
In use, the typical floating plate monitoring system provides two modes of operation, a xe2x80x9cdecayxe2x80x9d mode and a xe2x80x9cfloatxe2x80x9d mode. In the decay mode the ion collecting plate or surface associated with the monitor is pre-charged to a positive or negative voltage level of typically 1000 volts or more. The time required for the collecting plate to be discharged to a level of 10% of the starting value, i.e. 100 volts, by ion impingement from the ion field is measured by a timer in the monitor to indicate the quantity of either polarity of air ion associated with the ion field. This measurement is done with a pre-charge level of positive 1000 volts to indicate the quantity of negative air ions or with a pre-charge level of negative 1000 volts to indicate the quantity of positive air ions. In either case the time required for the plate to be discharged to a 100 volt level by the attraction of the oppositely charged air ion to the ion collecting plate is measured and used to indicate the xe2x80x9crichnessxe2x80x9d of the particular ion in the field.
In the float mode the ion collecting plate is initially pre-set to a zero voltage level and then allowed to xe2x80x9cfloatxe2x80x9d to a voltage level dictated by the impingement of incident ions from the ion field. The float measurement indicates the effectiveness of the field in reducing the net charge on the semiconductor devices to a low value while also indicating to what voltage level all devices, even devices which were not initially charged, will be driven to by the ion field.
Floating plate monitoring systems heretofore available suffer from many disadvantages. One results from the fact that the ion collecting plate element of the monitor system, in order to be isolated from ground by a high impedance, is typically monitored by use of a non-contacting electrostatic voltmeter probe device to indicate the voltage level of the plate element in reference to ground. These electrostatic voltmeter probes, typically of the field xe2x80x9cmillxe2x80x9d or tuning fork chopper types, are expensive and require a large mounting space at the ion collecting plate assembly to effectively read the plate voltage.
Another disadvantage is that to pre-charge the ion collecting plate for decay mode measurements, a relay or solenoid is typically used to momentarily connect the plate to a pre-charge level voltage source. This requires a low leakage relay scheme to maintain the plate at a high impedance level relative to ground thus typically requiring the relay to be positioned at the ion collecting plate structure itself.
A further disadvantage arises from the fact that as dictated by the measurement standards, the capacitance between the ion collecting plate and its associated ground referenced structures must be held at a specified capacitance of typically 20 pf+/xe2x88x9210%. This requires a specific plate geometry configuration that dictates a minimum volume ion collecting plate structure. This structure is typically large, in the order of 15 cmxc3x9715 cm with a thickness of 6 to 7 cm minimum. Using this type structure, it is not possible to position the ion collecting plate structure directly on line with the semiconductor devices for direct measurement of the effectiveness of the ion field at the location of the semiconductor devices themselves, therefore reducing the accuracy of the measurement.
Another disadvantage is that to operate the ion collection plate assembly a large diameter connecting cable to the monitor electronic assembly is typically used to accommodate the wiring for the electrostatic probe device electronics, the HV relay device actuation wiring, the high voltage wiring for the pre-charge supply and the ground reference connection for the ground reference element. This results in a bulky ion collecting plate assembly cable which limits assembly position flexibility and easy placement of the assembly.
A further disadvantage is that whereas it is desirable to construct an ion collection plate which is of the same relative size and thickness as the semiconductor devices which are placed within the ion field, the current art ion collecting elements, due to their required size to accommodate the required relay, electrostatic probe, and required measurement capacitance, cannot be constructed in the size desired.
Another disadvantage is that to produce the required ion collecting plate capacitance relative to the ground referenced element precise manufacturing of the ion collecting plate is required while other mechanical components such as spacers to support the ion collecting plate element from the ground reference element must be of high precision, low leakage and high cost construction.
A still further disadvantage is that the bandwidth (frequency response) of current art floating plate monitoring systems is limited to approximately 40 to 50 Hz due to the use of the typical electrostatic probe system used to monitor the ion collecting plate. This is a serious limitation particularly where A.C. type air ion production equipment is employed. The A.C. type ion production equipment can induce damaging A.C. fields and voltages at the location of the semiconductor devices and cannot be accurately measured using current art type floating plate monitors.
It is, therefore, a primary object of this invention to eliminate the aforementioned disadvantages found with floating plate monitoring systems heretofore available while reducing the cost of such systems.
It is a further object of this invention to eliminate the necessity and use of the typical electrostatic voltage probe in the ion plate assembly construction.
It is a further object of this invention to eliminate the necessity and use of a high voltage relay or solenoid in the ion plate assembly construction.
It is a further object of this invention to eliminate the requirement of providing an ion collecting plate structure which is dependent upon mechanical considerations such as plate area and plate spacing, both relative to the ground reference structure, to establish the specified ion plate capacitance to the ground reference, but to establish the specified capacitance using electrical means.
It is a further object of this invention to reduce the connecting cable to the ion collecting plate structure to a highly flexible small diameter cable to allow easy positioning of the ion plate assembly to the areas of measurement by the use of a single conductor shielded cable.
It is a further object of this invention to provide a high bandwidth capability floating plate voltage monitoring system to allow for the detection and measurement of A.C. electrical fields which are in the vicinity of the ion collector plate.
It is a further object of this invention to provide a technique whereby an isolated surface or structure may be charged to a given voltage level during a xe2x80x9cchargexe2x80x9d sequence while the voltage level of the isolated surface or structure can be monitored during a xe2x80x9creadxe2x80x9d sequence using a single shielded conductor to the floating plate assembly to charge, discharge and monitor the ion collection surface or structure.
It is a further object of this invention to provide a technique whereby an isolated surface or structure can be shielded from extraneous electrostatic charges, voltages, or fields on as many sides of the surface or structure as desired while maintaining very low capacitive loading or resistive loading due to leakage from the ion collecting surface to the shielding electrode or by the capacitance or leakage effects of the connecting cable.
It is a further object of this invention to provide a technique which will allow the ion collector plate assembly to be operated without the use of an electrostatic probe device, a relay device, and/or a bulky cable so as to allow the assembly to be constructed in various sizes and thicknesses which are comparable to the size and thickness of various semiconductor devices which are being processed to provide for high accuracy simulation by the floating plate monitoring system of the response of the semiconductor devices to the ion field and/or A.C. field.
It is a further object of this invention to provide a floating plate monitoring system which requires low maintenance and features high reliability by eliminating the use of mechanical devices such as electrostatic probes and relays or solenoids.
The present invention provides a floating plate ion monitoring system and method wherein an ion collector assembly comprises an ion conducting surface and shielding surface in spaced relation, the ion conducting surface being located to receive ion impingement thereon, and wherein a potential is applied to the shielding surface which duplicates and follows the voltage appearing on the ion conducting surface. As a result, the capacitance between the ion conducting surface and the shielding surface is established electrically and independent of the physical configuration of the ion collector assembly. The foregoing is accomplished by providing a unity gain connected operational amplifier and connecting the positive input thereof to the ion conducting surface and the inverting input thereof to the shielding surface. An indicator connected to the amplifier output monitors the voltage on the ion conducting surface. A voltage reference is applied to the electrically established capacitance so that the voltage rating thereof need not be equal to or greater than the limits of voltage appearing on the ion conducting surface. The magnitude of the applied voltage reference can be varied in a manner varying the magnitude of the electrically established capacitance.
The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the ensuing detailed description together with the included drawing.